The best cost driver that Carlson has for variable factory overhead in the assembly department is machine hours. During July, the company budgeted 720 machine hours and $6,480 for its Kentucky plant's assembly department. The actual variable overhead incurred was $6,870, which was related to 750 machine hours. The variable overhead spending variance and variable overhead efficiency variance are: Select one: a. Variable overhead spending variance $120 F, Variable overhead efficiency variance $270 U b. Variable overhead spending variance $60 U, Variable overhead efficiency variance $270 U c. Variable overhead spending variance $270 U, Variable overhead efficiency variance $60 U d. Variable overhead spending variance $120 U, Variable overhead efficiency variance $270 U